An oil mist separation mechanism of an internal combustion engine separating an oil mist included in blow-by gas is known. Such oil mist separation mechanism of the internal combustion engine is disclosed in, for example, JP2010-159733A.
In JP2010-159733A, a circulation apparatus of the internal combustion engine that circulates the oil separated from the blow-by gas into an oil pan is disclosed. Such circulation apparatus of the internal combustion engine includes an oil separator fixed to a lid portion covering an outer surface of an upper portion of a cylinder block, an oil circulation passage extending so as to be along an upper surface and a side surface of the cylinder block, and an oil drain pipe mounted on a bottom end of the oil circulation passage. Accordingly, the oil separated by the oil separator of the lid portion is returned to oil stored in the oil pan via the oil circulation passage and the oil drain pipe. Here, in the circulation device of the internal combustion engine of JP2010-159733A, the oil separator is separately provided relative to the lid portion.